Windows 8 and anti virus software

I've just replaced My Pc with one running Windows 8 pro.On previous machines I've had Avast installed, but Windows 8 seems to have decent anti virus protection included anyway.Can I rely on Windows 8 protection, or should I install Avast also?If I should have additional protection, is Avast the best option? I've gone with it in the past as it seems to be the one that slows the system down the least.Thanks for advice on this.--David